The present disclosure relates to electric partial discharge detection, and more particularly, to a system and method for detection and measuring electric partial discharge using an optical fiber.
As electrical wires age, insulation may become brittle and crack leading to arcing or shorts. Small arcs can carbonize the insulation, leading to bigger arcs and the potential for fire. Control of electrical discharge is necessary to avoid degrading the insulation in electrical wires in air or in insulating oil. The need for reliable monitoring is increased when the cable is used in high altitude where ambient pressures may be well below one atmosphere such at flight altitudes. The inception level for discharges is reduced as pressure decreases. Therefore, intermittent partial discharge or arcing that can occur in flight may not be reproducible during ground maintenance.
Intermittent discharges can occur from a wire with cracked or faulty insulation. Such partial discharges are in the form of pulses having time durations as short as a few nanoseconds or less. The pulses form because, once a discharge initiates, electrons quickly become depleted in the gap by buildup of space charge between the electrodes.
This extinguishes the discharge until the space charge dissipates sufficiently for a fresh discharge to occur. The presence of residual space charge along with surface charges and meta-stable excited gases act as a memory thus affecting ensuing pulse amplitudes and times of onset. Such discharges create an avalanche of electrons whose collisions with nitrogen molecules in air produce spectrum from near-UV photons to visible as shown in FIG. 1.
In order to detect the presence of cable defects in air or in oil, various imaging systems or electrical reflectometry techniques have been used to monitor entire wiring systems. But these devices provide limited coverage or require direct electrical connection, which can be inconvenient or even unsafe in some applications.
One potential detection concept that has been reported is photoluminescence. Photoluminescence is the emission of near or visible light from any substance under external excitation and it can be categorized into fluorescence and phosphorescence. The emission rates of fluorescence are typically 108/sec and its lifetime is near 10 nsec. Phosphorescence is emission of light from triplet-excited states, in which the electron in an excited orbital has the same spin orientation as the ground state electron. The transitions to ground are forbidden and the emission rates are slow (103-100), so phosphorescence lifetimes are typically milliseconds to seconds.
When an electron is excited to an upper state, it decays non-radiatively to a lower meta-stable state. From this meta-stable state the, electron decays radiatively emitting a photon. The energy of the emission, which is described as E=hν (where h is Plancks constant and ν is the frequency of the light) is typically less than that of absorption.
Sarkozi, U.S. Pat. No. 7,142,291, proposed a system for detecting partial discharges in wiring using a fluorescence optical fiber. The system required large diameter plastic core fiber optic cables embedded into the insulation material surrounding the wiring. The large core diameter optical fiber had a fluorescent material incorporated into the fiber core. Upon exposure of that fluorescent material to a discharge frequency it generates light that travels in both directions in the optical fiber. Optical detectors at each end of the fiber record the arrival of that light and a processor calculates a location of the discharge from the arrival times.
McSherry, et. al. (IEEE Sensors Journal October 2004) proposed a coated sensor model for measuring germicidal UV irradiation in which a large core diameter plastic optical fiber had its cladding removed and replaced by a fluorescent coating. When placed near a UV lamp the fluorescent coating would generate light that would travel down the optical fiber core to be detected.
Both of these prior art systems made use of large (1000 micro-meter) core diameter plastic optical fibers. Plastic fibers usually have cores of polymethylmethacrylate (PMMA) and typically have core diameters close to 1000 micro-meters. Such fibers have a limited transmission distance and poorer resistance to environmental conditions than optical fibers of a silica (glass) core. Silica glass core fibers typically have silica cores of between 50-500 micro-meters. Thus the plastic core fibers of these prior art references are significantly limited to very short sensor systems (around 10 m range) in favorable environments. In addition less complex detection systems than the two optical detector system of Sarkozi are needed. A silica core based sensor system with a hard plastic cladding for electric partial discharge would enable sensor systems based on large distances (potentially up to 1 kilometer with attenuation of less than 1 dB/100 m at the 550 nm emission band) and in more stringent environmental conditions.
In addition there is a need is for a distributed system that is easily deployable in close proximity to the objects being monitored without the need to incorporate them into the structural design as in the Sarkozi reference. And there is a need to decrease bend related attenuation in the fibers, which is possible with much smaller (<500 micro-meter) core diameter fibers with silica cores.